Field of the Invention
The present invention relates to a method for driving a switch in a switch-mode converter and also relates to a drive circuit for a switch in a switch-mode converter. The method includes: producing a threshold signal, producing a ramp signal synchronized with a clock signal such that the slope of the ramp signal is dependent on the output voltage of the converter, and comparing the threshold signal with the ramp signal and driving the switch as a function of the result of the comparison.
A method such as this and an apparatus such as this are known from U.S. Pat. No. 6,307,361. In the known method and the known apparatus, a signal that is dependent on an input current to a switch-mode converter is compared with a pulsed ramp signal or a sawtooth waveform signal. The pulsed ramp signal or the sawtooth waveform signal has a slope that is dependent on the output voltage of the switch-mode converter. A switch, which controls the power consumption of the switch-mode converter is in this case pulsed, and is driven as a function of a comparison of the threshold signal with the ramp signal. The switch is switched on when the ramp signal reaches the value of the threshold signal and is switched off when the ramp signal is reset. When the input voltage of the switch-mode converter rises during this process, then the period for which it is switched on decreases, in order to keep the power consumption constant. When the output voltage falls because of an increase in the power consumption of any load that is connected to the switch-mode converter, then the period for which it is switched on increases, in order to increase the power consumption, and thus to readjust the output voltage.
In the case of the known method and the known apparatus, the input current and hence the threshold signal are inversely proportional to the square of the input voltage. The input voltage is normally a sinusoidal voltage or a voltage with a sinusoidal magnitude, at a frequency that is considerably lower than the clock frequency of the switch. If the root mean square value of the input voltage varies between 90 V and 270 V (=3xc2x790 V), as is the case for switch-mode converters in so-called wide area network sections, then, because of the input voltage, this results in a dynamic range for the threshold value of 1:32, or 1:9. Furthermore, if the output voltage is constant, the input current is proportional to the power emitted to the load. If, by way of example, this power consumption varies by a factor of 1:20, then these fluctuations result in the threshold value having a dynamic range of 1:20. In other words: when the input voltage is at its maximum root mean square value and the power consumption of the load is at its minimum, which is then the minimum threshold value xmin, then the maximum threshold value which occurs with the input voltage with the lowest root mean square value and with the maximum load power consumption is xmax=180xc2x7xmin.
The circuit configuration for producing the ramp signal must in this case be designed such that it produces a ramp signal which has just as wide a dynamic range, in order on the one hand to reach the threshold value in each clock period, and hence to switch on the switch. Furthermore, the comparator must be designed to produce an exact comparison result over this dynamic range.
The switch-mode converter taught in U.S. Pat. No. 6,307,361 is in the form of a boost converter, whose power consumption increases as the period for which the switch is switched on in each clock period increases.
A method for driving a switch in a switch-mode converter in the form of a boost converter is known from Sam Ben-Yaakov, Ilya Zeltser: xe2x80x9cPWM Converters with Resistive Inputxe2x80x9d, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 45, NO. 3, June 1998, in which the input current on the network side is filtered and is multiplied by a controlled variable that is dependent on the output voltage. The multiplication signal that is formed is compared with a ramp signal or sawtooth waveform signal, in order to define periods during which the switch in the switch-mode converter is switched off, on the basis of the comparison result. In this method, when the input voltages have a high root mean square value and the loads are low, the controlled variable that is dependent on the output voltage would have to be very large, so that a multiplier with a very wide linearity range is required, and this can be produced only with a high degree of circuitry complexity.
A method is likewise known from Hwang, Chee, Ki: xe2x80x9cNew Universal Control Methods for Power Factor Correction and DC to DC Converter Applicationsxe2x80x9d, IEEE, 1997, in which a threshold signal which is dependent on an input current to a switch-mode converter is compared with a ramp signal whose amplitude and slope are variable. The switch in the switch-mode converter is driven as a function of a comparison of the threshold signal with the ramp signal. In this method as well, the amplitude of the variable ramp signal would need to have a very wide dynamic range in order to reach the threshold signal, and hence to switch on the switch, once in each clock period. Furthermore, there are stringent requirements for the accuracy of the comparison configuration that compares these two signals, namely the threshold signal and the ramp signal, which have a wide dynamic range.
A method for driving a switch in a switch-mode converter is known from Published German Patent Application DE 10 725 842 A1, in which the instantaneous value of the output voltage is multiplied by a controlled variable that is dependent on the output voltage. A difference signal between the multiplication signal and a signal that is dependent on the input current is supplied to a pulse width modulator in order to produce drive signals for the switch. An exponential transfer function is in this case applied to the control signal before it is supplied to the multiplier.
It is accordingly an object of the invention to provide a method and a drive circuit for driving a switch in a switch-mode converter, which overcome the above-mentioned disadvantages of the prior art apparatus and methods of the general type in which a threshold signal is compared to a ramp signal that is dependent on an output voltage of the switch-mode converter, and in which the switch in the switch-mode converter is driven as a function of the result of the comparison.
In particular, an object of the invention is to provide a method and a drive circuit of the type mentioned above that can be used in switch-mode converters having a predetermined dynamic range for the input voltage and having a predetermined dynamic range for the power consumption of the load.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for driving a switch in a switch-mode converter having input terminals for supplying an input voltage and output terminals for providing an output voltage. The method includes the following steps: producing a threshold signal dependent on a quotient of a first signal dependent on an input current to the switch-mode converter and a second signal dependent on the output voltage; producing a ramp signal synchronously in time with a clock signal, the ramp signal having a slope dependent on the output voltage; and obtaining a comparison result by comparing the threshold signal with the ramp signal and driving the switch as a function of the comparison result.
Using a threshold signal that is dependent on the quotient of a signal, which is dependent on the input current, and a signal, which is dependent on the output voltage, reduces the dynamic range of the threshold signal. This results in the ramp signal likewise having a narrower dynamic range, thus also reducing the requirements for a comparator circuit that carries out a comparison between the threshold signal and the ramp signal.
One embodiment of the method provides for a control signal that is dependent on the output voltage to be produced, with the second signal (which is dependent on the output voltage) being related to this control signal via a nonlinear characteristic. This nonlinear characteristic is preferably an exponential characteristic. This improves the overall control response, particularly when major changes occur in the input voltage, as has already been described in principle in Published German Patent Application DE 197 25 842 A1.
One embodiment of the invention also provides for the slope of the ramp signal to be related to the control signal (which is dependent on the output voltage) via a nonlinear characteristic, preferably an exponential characteristic, and this is likewise used to improve the overall control response.
The control signal is preferably formed by forming the difference between a reference voltage signal and a signal that is proportional to the output voltage, and by subsequently filtering the difference signal. The regulator that is used to form a control signal such as this is preferably a proportional regulator or a proportional integral regulator.
The second signal, which has a reciprocal to which the threshold signal is proportional, and the slope of the ramp signal can be related to this control signal via the same characteristic.
Another embodiment of the invention provides for the second signal and the slope of the ramp signal to be related to the control signal via different characteristics, with the product of these two characteristics preferably producing an exponential characteristic.
The method is used in particular for driving a switch in a switch-mode converter in the form of a boost converter. The switch preferably is switched on when the ramp signal reaches the threshold signal, and is switched off synchronously in time with a clock signal that governs the period duration of the individual ramps. In the case of a boost converter, the power consumption increases as the time for which this switch is switched on increases. Conversely, the power consumption falls when the switch is in each case closed for only a fraction of the period duration during each clock period. In the context of the present invention, the expression xe2x80x9cramp signalxe2x80x9d refers to a signal that rises periodically at the start of a period duration, or after a predetermined time following the start of the period duration, and which is reset at the end of the period duration to an initial value. Short switched-on times are in this case achieved when the slope of the ramp signal and the amplitude value of the threshold signal are matched to one another such that the ramp signal does not reach the threshold value until shortly before the end of the period duration. That is to say, when there is little difference between the amplitude of the threshold signal and the maximum amplitude value of the ramp signal that can be reached during one period. If parasitic effects result in minor fluctuations in the threshold signal when the power consumption is low, then it is possible for the switch to be closed too early, or not at all, during one period duration, which is detrimental to the control of the output voltage. In order to prevent this, one embodiment of the invention provides for the slope of the ramp signal to increase for a predetermined time period before the resetting of the ramp signal, in order in this way to ensure that the ramp signal reaches the value of the threshold signal, preferably at the end of a clock period, so that there is a greater probability of the switch finally being switched on within each clock period. The increase in the slope of the ramp signal shortly before the resetting of the ramp signal may in this case be predetermined to be constant, or may be dependent on the output voltage or on a control signal that is dependent on the output voltage.
Before the comparison with the ramp signal, the threshold signal is preferably subjected to low-pass filtering with a cut-off frequency that is dependent on the second signal. This means that the power factor correction of the switch-mode converter is improved when the power consumption is low, when the switch-mode converter is operating in the discontinuous current mode (DCM), and that it is possible for the current control loop to achieve a high degree of stability when the power consumption is high, when the switch-mode converter is operating in the continuous current mode (CCM).
The drive circuit for a switch in a switch-mode converter has a threshold signal production circuit which is in the form of a divider circuit. The threshold signal production circuit is supplied with a first signal that is dependent on an input current to the switch-mode converter and on a second signal that is dependent on the output voltage of the switch-mode converter, and the threshold signal production circuit produces an output signal which is dependent on the quotient of the first signal and of the second signal. The drive circuit also has a ramp signal production circuit, which produces a ramp signal in time with a clock signal, with the slope of the ramp signal being dependent, at least in places, on the output voltage, and a comparator circuit to which the threshold signal and the ramp signal are supplied and which produces an output voltage as a function of which the switch is driven.
In one exemplary embodiment, the divider circuit includes a low-pass filter with a cut-off frequency that is dependent on the second signal, and at whose output the threshold signal is produced. This low-pass filter makes it possible to positively influence the power factor correction of the switch-mode converter when the power consumption is low, and to positively influence the control stability of the current control loop when the power consumption is high.
A current detection resistor is preferably provided for producing a signal that is dependent on the input current to the switch-mode converter. This current detection resistor is connected in the input circuit of the switch-mode converter, and one of its connections is connected to the divider circuit. The divider circuit preferably includes a multiplier, to one of whose inputs the second signal is supplied and to whose other input the threshold signal is supplied. The multiplier produces at its output, a current that is dependent on the product of the second signal and of the threshold signal. The divider circuit also has a resistor, which is connected between the output of the multiplier and the current detection resistor, and a differential amplifier. One of the inputs of the differential amplifier is connected to the output of the multiplier, and the other input is connected to a reference ground potential. The output of the differential amplifier provides the threshold signal. The reference ground potential is preferably also the potential with respect to which the output voltage is produced and to which that connection of the current detection resistor that is remote from the divider circuit is connected. The current detection resistor, the multiplier with the downstream resistor and the differential amplifier operate as a control loop, with the current which is supplied from the multiplier always being set such that the voltage drop across the resistor that is connected downstream from the multiplier corresponds to the voltage drop across the current detection resistor, so that the current which is supplied by the multiplier is proportional to the input current to the switch-mode converter. If the voltage drop across the current detection resistor and the output current of the multiplier are regarded as a first signal, then this first signal is obtained from the product of the threshold signal and the second signal, and the threshold signal is then obtained from the quotient of the first signal and of the second signal.
In one embodiment of the drive circuit, the output of the differential amplifier is fed back via a capacitive circuit to the input of the differential amplifier, which is connected to the output of the multiplier. This results in a low-pass response with a cut-off frequency that is dependent on the second signal.
In one embodiment of the invention, a filter having a nonlinear characteristic is provided, to whose input, a signal which is dependent on the output voltage is supplied and at whose output the second signal is available. This filter preferably has an exponential characteristic.
The signal that is dependent on the output voltage and is supplied to the filter, or which can be supplied to the divider directly as a second signal and to the ramp signal production circuit indirectly in order to adjust the slope of the ramp signal, is preferably available at the output of a regulator that is in the form of a proportional regulator or proportional integral regulator, and to which a voltage signal which is proportional to the output voltage is supplied.
In a further embodiment, a filter is provided to which a signal that is dependent on the output voltage, in particular the signal which is produced at the output of the regulator, is supplied and which produces the second signal and a slope signal. The slope signal is in this case supplied to the ramp signal production circuit in order to adjust the slope of the ramp signal. The product of the second signal and the slope signal is related to the filter input signal via a nonlinear characteristic, preferably an exponential characteristic.
The provision of filters such as these with nonlinear characteristics improves the overall control response of a switch-mode converter with the inventive drive circuit, in particular when sudden changes occur in the input voltage to the switch-mode converter.
The oscillator signal is supplied by an oscillator circuit. The ramp signal production circuit produces the ramp signal synchronous in time with the oscillator signal. This oscillator circuit preferably produces a second oscillator signal, which is supplied to the ramp signal production circuit. The ramp signal production circuit is designed such that the slope of the ramp signal can be increased synchronously with the second oscillator signal, thus making it possible to ensure that the switch is closed once in each clock period even when the power consumption is low, which has a positive effect on the stability response of a switch-mode converter with the drive circuit according to the invention.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for driving a switch in a switch-mode converter, and a drive circuit for driving a switch, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.